International Application WO 00/02417 describes a loudspeaker comprising a visual display screen, a panel-form member positioned adjacent to the display screen and at least a portion of which is transparent and through which the display screen is visible, and a vibration exciting transducer mounted to an edge or marginal portion of the panel-form member to apply energy to the panel-form member to cause the panel-form member to act as an acoustic radiator, characterized in that the panel-form member is arranged to be resonant at audio frequencies, in that the vibration transducer is adapted to apply bending wave energy to the panel-form member to cause it to resonate to act as an acoustic radiator when resonating and in that one or more marginal portions of the panel-form member are clamped or restrained. This arrangement has a number of advantages, including:
1) Minimizing the footprint of the loudspeaker in a given application.
2) Improved user experience, where the image and sound come from the same location.
3) Ability to reproduce stereo from two spatially separated channels on the same plate.
International Application WO 99/37121 describes methods for excitation of a panel-form bending wave radiator, e.g. a transparent plate, including choice of exciter location to optimize the distribution of excited modes for a smooth transfer of energy.
The design of a display system based on this prior art can be limited at low frequency for the following two reasons:
1. The low frequency limit for useful radiation from the plate is determined by the gap between the plate and the screen. The cavity formed behaves as a distributed compliance, which together with the areal density of the plate forms a mass spring resonance. Below this resonance frequency the modes excited in the plate radiate only weakly, whereas above this frequency useful modal radiation may be achieved, and
2. A second parameter that controls the effective low frequency limit for the system is the visibility of vibrations on the plate. For a high quality visual display visual vibration can be unacceptable. The most dominant effect is the visibility of reflections from the plate, rather than any disturbance of the direct image of the screen. This may be minimized with control over the environment in which the unit is used, such as lowering the light level in the room, or angling the screen to minimize the visibility of reflections from light sources in the room. Anti-reflection coatings on the plate may improve the performance. In many applications, however, there is no direct control over these environmental factors, and this problem can be severe.
The visibility of vibration gives an alternative low frequency limit for the useful bandwidth over which a transparent loudspeaker in front of a screen may be used. The limit is manifest as a maximum level at a given low frequency energy. The limit becomes more severe at progressively low frequencies below approximately 250 Hz. The figure of ˜250 Hz is controlled by the sensitivity of the human visual system. The vibrations above this frequency are progressively lower in amplitude (for a given SPL) and vary at a higher rate. The human visual system averages out these fluctuations and the visibility of vibration is markedly reduced.
The prior art therefore discloses a transparent loudspeaker, optimized for its distribution of excited modes, which provides a high quality sound output above a low frequency limit. The low frequency limit is determined both by the depth of the cavity and the visibility of vibration. This limits the useful sound output to ˜250 Hz for display systems of the highest quality.